(1) Field of the invention
The present invention generally relates to an ink jet recording method and head, and more particularly to an ink jet recording method and head in which a dot is recorded using one or a plurality of ink droplets so that the size of the dot is controlled.
(2) Description of the related art
A non-impact recording method is advantageous since a noise level generated during a recording process is low enough to be ignored. Particularly, an ink jet recording method, which is one example of the non-impact recording method, can make prints at a high velocity and can make prints on normal sheet without an image fixing process. Since, the ink jet recording method is a very useful recording method, printers using the ink jet recording method have been proposed and have been put into practical use.
In such an ink jet recording method, droplets of recording liquid named as ink are jetted, the ink droplets are adhered to the recording medium and images are formed on the recording medium by the adhered ink droplets. The ink jet recording method is disclosed, for example, in Japanese Patent Publication No.56-9429. In the method disclosed therein, a bubble is generated in the ink in a liquid chamber by heating the ink so that pressure in the ink is increased. The ink is then jetted, as an ink droplet, from a fine orifice at the lead end of a nozzle and an ink dot is recorded on the recording medium.
Various method have been proposed based on the above principle of the ink jet recording method. For example, Japanese Laid Open Patent Application No.59-207265 discloses a method by which-gray scale images are recorded. In this method, a sequence of pulses is supplied to a heater so that ink droplets are generated, a single droplet into which the generated ink droplets are connected is jetted to a recording medium, and a single dot is formed on a recording medium. The number of the generated ink droplets is controlled in accordance with the number of pulses included in a sequence of pulses.
A method disclosed in Japanese Laid Open Patent Application No.63-53052 has been known. In this method, a gray scale image is recorded by jetting a sequence of ink droplets which are to be fused into a single dot on a recording medium within a wet time of the recording medium. That is, ink droplets are separately jetted at a high velocity and reached to a recording medium, and the ink droplets are then fused into a single dot on the recording medium within the wet time of the recording medium. The size of the dot on the medium corresponds to the number of ink droplets fused into the single dot within the wet time of the recording medium.
Further, a method disclosed in Japanese Patent Publication No.59-43312 has been known. In this method, to improve the output responsibility and stability of ink droplets in response to pulses supplied to a heater to generate bubbles in the ink, an input interval of the pulses in the maximum frequency at which ink droplets are generated is controlled so as to be as large at least three times as the half-width of each pulse.
In the method disclosed in Japanese Laid Open Application No.59-207265, to maintain a condition in which a plurality of jetted ink droplets are connected together to form a single ink droplet, the ink droplets must be jetted at a low velocity. However, if the droplets are jetted at the low velocity, a locus in which each droplet is jetted is not stable, so that deterioration in the quality of prints occurs. In addition, the ink droplets jetted at the low velocity are easily affected by the malfunction of the ink jet recording head and the variation in the moving velocity of the recording head. If the ink jet recording head is moved at a high velocity, a true circular dot is not made on the recording medium when the jetted ink droplets are adhered to the recording medium. As a result, an image formed on the recording medium becomes not clear.
Japanese Laid Open Patent Application No.63-53052 does not disclose conditions under which ink drops are-to be jetted other than only a condition in which a time interval separating the activation of the heater to jet the next ink droplet from the disappearance of the bubble falls within a range between 0.1 microsecond and 1.0 millisecond. Thus, it can not be understood under what conditions ink droplets are to be jetted nor how the recording head to be used is to be structured, so that the method can not realized.
Japanese Patent Publication No.59-43312 describes only conditions under which ink droplets can be stably jetted by an on-off operation of a pulse signal. That is, the gray scale printing method is not disclosed in Japanese Patent Publication No.59-43312, but discloses only conditions for a stable binary printing operation.